1. Field of the Invention
The present invention relates to an electrode unit joining structure for a superconducting wire, a superconducting wire, and a superconducting coil.
2. Description of the Related Art
In recent years, various equipment devices as electric power application equipment using a superconducting wire that is obtained by processing a superconductor into the wire rod have been developed. For example, a superconducting magnetic energy storage (hereinafter, simply referred to as “SMES”) has characteristics such as an energy input and output speed that is faster than other energy storages, and thus has been actively developed. In addition, development of an AC coil represented by a transformer, a superconducting rotator represented by a motor or a power generator, a fault current limiter that uses a wire rod that is highly resistive during normal conduction, and the like have been progressing. As this superconductor, metal-based superconductors or oxide-based superconductors have been known.
When not cooled to an extremely low temperature such as approximately 4.2 K, metal-based superconductors such as NbTi and Nb3Sn may not obtain a superconducting state, and thus the cooling cost is high and so these conductors are not put into practical use. In addition, it is known that when a current exceeding a critical current flows, normal conduction transition is caused instantaneously in the metal-based superconductors, and thus the metal-based superconductors may not maintain a superconducting state. Therefore, when the above-mentioned metal-based superconductor is processed into a wire rod to form a coil body and this coil body is applied to the SMES, normal conduction transition occurs instantaneously in a wire rod located at a position through which a current exceeds a critical current, and as a result, there is a problem in that energy stored in the coil is released.
On the other hand, in the case of using a Bi (bismuth)-based or Y (yttrium)-based oxide superconducting wire, a superconducting transition temperature of the superconducting wire is relatively high, and liquid nitrogen at 77.3 K may be used as a cooling material that causes the wire rod to be in a superconducting state, and thus the cooling cost may be greatly reduced. In addition, in the case of the Bi-based or Y-based oxide superconducting wire, even when a current exceeding the critical current is made to flow, when it is within a temperature range called a magnetic flux region, the normal conduction transition is not caused to occur and the superconducting state may be maintained. Therefore, an application utilizing an advantage of this oxide superconducting wire has been anticipated.
In recent years, in a tape-shaped Y-based oxide superconducting wire, a wire rod, which has a superconducting characteristic in which the critical current is approximately 100 A to 300 A with a tape width of 5 mm when the wire rod is cooled to 77.3 K by liquid nitrogen and an external magnetic field is set to 0 T, has become available. This oxide superconducting wire may be used, for example, as a pancake type coil (a pancake coil) or the like. This pancake coil is a coil that is formed by concentrically winding the tape-shaped oxide superconducting wire around a cylindrical winding frame, and a superconducting coil, to which external connection electrodes are attached at a leading end portion and a trailing end portion of the superconducting wire, respectively, has been disclosed (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2008-140930 to be described below).
The superconducting coil that is disclosed in Japanese Unexamined Patent Application, First Publication No. 2008-140930 is a pancake coil that is formed by winding a thin film superconducting wire having a multi-layer structure in which one surface side is a superconducting layer side and the other surface side is a substrate side. An inner electrode is provided on an inner circumferential surface at a leading end of the innermost turn and an outer electrode is provided on an outer circumferential surface at a leading end of the outermost turn of the thin film superconducting wire, respectively.
However, when energization is carried out with respect to the superconducting coil having a structure disclosed in Japanese Unexamined Patent Application, First Publication No. 2008-140930 through an electrode from an external excitation power supply, due to an increase in temperature, which is caused by heat permeating from the outside and heat dissipation at a connection portion, normal conduction transition in which a superconductor in the vicinity of the electrode transitions from a superconducting state to a normal conduction state occurs, and thus there is a problem in that burning-out of the superconducting coil may occur.